The present invention relates to converter circuits, and in particular, to a circuit for ensuring that sufficient voltage is provided for operating a current sense amplifier circuit which senses the output current of the converter. This is particularly important in multi-phase converters where there are plural phases each sharing in the output load.
In a multi-phase converter, plural converters are coupled together at an output node to provide an output voltage. In a buck converter topology, for example, each converter comprises two series connected switches connected across a voltage bus. The switches are alternately controlled, typically by pulse width modulated signals, to ensure that a desired output voltage appears at the output node. The output voltage is regulated by a feedback loop. Typically, each converter is operated in a phased relationship with the other phases, so at any instant in time, one control switch of the converter is on. In a buck converter topology, the switched node between the two switches is coupled to the output node by an inductor. The energy stored in the inductor is coupled to an output storage capacitance. Each of the plural switching converters is coupled through a respective inductor to the output capacitance. The load is connected across the output capacitance.
In such a multi-phase converter, it is important that each phase or channel (switching converter) provide an equal share of the total load current or poor efficiency will occur. It is also possible that the power supply will be damaged if a phase provides too much current in comparison to the other phases. One way to ensure that the phases share equally in the overall current load is to sense the current in each phase, i.e., the inductor current in each phase and use that current information to force current sharing by modulating individual converter phase duty cycles. The current information needs to be accurate and is a critical parameter.
Inductor current can be sensed by connecting a network comprising a series resistor and a capacitor in parallel with the inductor and measuring the voltage across the capacitor. This current sensing circuit is shown in FIG. 2. Usually the resistor RCS and the capacitor CCS are chosen so that the time constant of RCS and CCS equals the inductive time constant which is the inductance L divided by the inductor direct current resistance (DCR) RL or the inherent resistance of the inductor, i.e., L/RL. If the two time constants match, the voltage across CCS is proportional to the current through the inductor and the sense circuit can be treated as if only a sense resistor with the value of RL was used. A mismatch of the time constants does not affect the measurement of inductor DC current but affects the AC component of the inductor current.
An advantage of sensing the inductor current in this way versus high side or low side switch sensing is that actual output current being delivered to the load is obtained rather than peak or sampled information about the switch currents. The output voltage can be positioned to meet a load line based on real time information. This method is also preferable to using a sense resistor in series with the inductor because this results in greater losses.
The current sense amplifier (CSA) shown in FIG. 2 operates with a common mode input equivalent to the output voltage VOUT where VOUT is the regulated voltage provided to the load. The current sense amplifier (CSA) inputs may eventually run out of headroom as the regulated voltage VOUT is increased. This is because the output voltage VOUT, if it is increased too much, approaches the voltage VCC powering the current sense amplifier. The current information reported by the current sense amplifier operating with inadequate headroom is useless and could be potentially grossly incorrect. The result is that current sharing does not occur or the sense circuit will force incorrect current sharing to the point of destruction of the converter or a portion of the converter.
As an example, in the current sense circuit of FIG. 2, the current sense amplifier is powered by voltage VCC which is typically coupled to an undervoltage lockout (UVLO) circuit which, as an example, will prevent operation of the circuit at a minimum VCC of 6.5 volts. Thus, the UVLO circuit will shut down the converter if VCC goes under 6.5 volts. If VOUT, for example, is set to 5.5 volts, this results in only one volt of current sense amplifier headroom. In a typical application, the nominal headroom requirement is 1.75 volts, 0.75 volts higher than the one volt headroom in this particular example, for proper current sense amplifier operation. One way to correct this is to increase the VCC UVLO threshold to a higher voltage, but this is not an adequate solution because it is desirable to have the ability to operate the converter at low VCC voltages to provide low VOUT voltages such as 1.2 volts where the current sense amplifiers will have sufficient headroom. So, because the circuit must be capable of being used with low VCC voltages, increasing the UVLO threshold is not an option.
It is desirable to provide a circuit which will allow the current sense amplifier to have adequate headroom voltage, i.e., the difference between the VCC voltage and the output voltage of the converter, and which does not require the UVLO threshold to be increased.